/*
 * Copyright (c) 2012, Metron, Inc.
 * All rights reserved.
 *
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 * modification, are permitted provided that the following conditions are met:
 *     * Redistributions of source code must retain the above copyright
 *       notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above copyright
 *       notice, this list of conditions and the following disclaimer in the
 *       documentation and/or other materials provided with the distribution.
 *     * Neither the name of Metron, Inc. nor the
 *       names of its contributors may be used to endorse or promote products
 *       derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL METRON, INC. BE LIABLE FOR ANY
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package com.metsci.glimpse.util.math.stochastic;

/**
 *  Modified from a C-program for MT19937 by Takuji Nishimura and Makoto Matsumoto.
 *  (http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/VERSIONS/C-LANG/mt19937-64.c)
 *  License on original C-program is 3-clause BSD.
 *  
 *  Seeding could be improved to use longs.
 *
 *  @author osborn
 */
public final class StochasticEngineMersenne implements StochasticEngine
{
    private final Generator _generator;

    private final static int N = 624;
    private final static int M = 397;
    private final static int MATRIX_A[] = { 0x0, 0x9908b0df };
    private final static int UPPER_MASK = 0x80000000;
    private final static int LOWER_MASK = 0x7fffffff;

    // state
    private transient int[] mt;
    private transient int mti;

    private StochasticEngineMersenne( int seed )
    {
        setSeed( seed );
        _generator = new GeneratorImpl();
    }

    public final State getState()
    {
        return null;
    }

    public final Generator getGenerator()
    {
        return _generator;
    }

    public static StochasticEngineMersenne createEngine( int seed )
    {
        return new StochasticEngineMersenne( seed );
    }

    private final void setSeed( int seed )
    {
        mt = new int[N];
        mt[0] = seed;

        for (mti = 1; mti < N; mti++)
            mt[mti] = (1812433253 * (mt[mti-1] ^ (mt[mti-1] >>> 30)) + mti);
    }

    protected final int next(int bits)
    {
        int y, kk;
        if (mti >= N) // generate N words at one time
        {
            for (kk = 0; kk < N-M; kk++)
            {
                y = (mt[kk] & UPPER_MASK) | (mt[kk+1] & LOWER_MASK);
                mt[kk] = mt[kk+M] ^ (y >>> 1) ^ MATRIX_A[y & 0x1];
            }

            for (;kk < N-1; kk++)
            {
                y = (mt[kk] & UPPER_MASK) | (mt[kk+1] & LOWER_MASK);
                mt[kk] = mt[kk+(M-N)] ^ (y >>> 1) ^ MATRIX_A[y & 0x1];
            }

            y = (mt[N-1] & UPPER_MASK) | (mt[0] & LOWER_MASK);
            mt[N-1] = mt[M-1] ^ (y >>> 1) ^ MATRIX_A[y & 0x1];

            mti = 0;
        }

        y = mt[mti++];

        // Tempering
        y ^= (y >>> 11);
        y ^= (y << 7) & 0x9d2c5680;
        y ^= (y << 15) & 0xefc60000;
        y ^= (y >>> 18);

        return (y >>> (32-bits));
    }

    private final class GeneratorImpl implements Generator
    {
        public final double nextDouble()
        {
            return (((long)next(26) << 27) + next(27)) / (double)(1L << 53);
        }

        public final int nextInt( int n )
        {
            // powers of two
            if ((n & -n) == n)
            {
                return (int)((n * (long)next(31)) >> 31);
            }

            int bits, val;
            do
            {
                bits = next(31);
                val = bits % n;
            }
            while(bits - val + (n-1) < 0);
            return val;
        }
    }
}
